The proposed studies are intended to delineate the role of GnRH in the secretion of pituitary gonadotropins and gonadal steroids. The mechanisms of action of GnRH on the pituitary and the gonad will be determined in vivo, and in vitro using both isolated cells and plasma membrane preparations. The interactions of GnRH with the pituitary plasma membrane receptor will be examined in detail and the nature of factors which control GnRH receptor number elucidated. GnRH receptors will be assessed using the newly developed assay which employs iodinated superagonist GnRH analogs as ligand. Subsequently, the role of GnRH receptors in determining gonadotropin response will be evaluated. The dynamic aspects of GnRH induced gonadotropin secretion will be investigated using isolated pituitary cells in a perifusion system. Cells will be exposed to pulsatile stimulation by GnRH and the effects of gonadal steroids on LH and FSH secretion evaluated. Efforts will be made to determine the source of GnRH which may play a role in modulating gonadal steroidogenesis and the mechanisms involved in the inhibitory effects of GnRH on ovarian and testicular steroid hormone secretion.